<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE article  PUBLIC "-//NLM//DTD Journal Publishing DTD v3.0 20080202//EN" "http://dtd.nlm.nih.gov/publishing/3.0/journalpublishing3.dtd"><article xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" dtd-version="3.0" xml:lang="en" article-type="research article"><front><journal-meta><journal-id journal-id-type="publisher-id">IJG</journal-id><journal-title-group><journal-title>International Journal of Geosciences</journal-title></journal-title-group><issn pub-type="epub">2156-8359</issn><publisher><publisher-name>Scientific Research Publishing</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="doi">10.4236/ijg.2024.151003</article-id><article-id pub-id-type="publisher-id">IJG-130907</article-id><article-categories><subj-group subj-group-type="heading"><subject>Articles</subject></subj-group><subj-group subj-group-type="Discipline-v2"><subject>Earth&amp;Environmental Sciences</subject></subj-group></article-categories><title-group><article-title>
 
 
  Estimation of Displacement and Extension due to Reverse Drag of Normal Faults: Forward Method
 
</article-title></title-group><contrib-group><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Shunshan</surname><given-names>Xu</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Angel</surname><given-names>Francisco Nieto-Samaniego</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Huilong</surname><given-names>Xu</given-names></name><xref ref-type="aff" rid="aff2"><sup>2</sup></xref></contrib><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Susana</surname><given-names>Alicia Alaniz-Álvarez</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib></contrib-group><aff id="aff1"><addr-line>Universidad Nacional Autónoma de México, Centro de Geociencias, Apartado Postal 1-742, Querétaro, Qro., México</addr-line></aff><aff id="aff2"><addr-line>South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China</addr-line></aff><pub-date pub-type="epub"><day>26</day><month>01</month><year>2024</year></pub-date><volume>15</volume><issue>01</issue><fpage>25</fpage><lpage>39</lpage><history><date date-type="received"><day>30,</day>	<month>October</month>	<year>2023</year></date><date date-type="rev-recd"><day>28,</day>	<month>January</month>	<year>2024</year>	</date><date date-type="accepted"><day>31,</day>	<month>January</month>	<year>2024</year></date></history><permissions><copyright-statement>&#169; Copyright  2014 by authors and Scientific Research Publishing Inc. </copyright-statement><copyright-year>2014</copyright-year><license><license-p>This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/</license-p></license></permissions><abstract><p>
 
 
  In the case of reverse drag of normal faulting, the displacement and horizontal extension are determined based on the established equations for the three mechanisms: rigid body, vertical shear and inclined shear. There are three sub-cases of basal detachment for the rigid body model: horizontal detachment, antithetic detachment and synthetic detachment. For the rigid body model, the established equations indicate that the total displacement on the synthetic base (
  D
  <sub>t2</sub>
  ) is
   the
   largest, that on the horizontal base (D<sub>t1</sub>) is moderate, and that on the antithetic base (D<sub>t3</sub>) is 
  the 
  smallest. On the other hand, the value of (D<sub>t1</sub>) is larger than the displacement for the vertical shear (D<sub>t4</sub>). The value of (D<sub>t1</sub>) is larger than or less than the displacement for the inclined shear (D<sub>t5</sub>) depending on the original fault dip δ<sub>0</sub>, bedding angle θ, and the angle of shear direction
   
  β
  . For all original parameters, the value of D<sub>t5</sub> is less than the value of D<sub>t4</sub>. Also, by comparing three rotation mechanisms, we find that the inclined shear produces largest extension, the rigid body model with horizontal detachment produces the smallest extension, and the vertical shear model produces moderate extension.
 
</p></abstract><kwd-group><kwd>Fault Rotation</kwd><kwd> Fault Drag</kwd><kwd> Fault Displacement</kwd><kwd> Extension</kwd><kwd> Forward Model</kwd></kwd-group></article-meta></front><body><sec id="s1"><title>1. Introduction</title><p>Anderson’s model (1951) [<xref ref-type="bibr" rid="scirp.130907-ref1">1</xref>] predicts dips of normal faults larger than 45˚ [<xref ref-type="bibr" rid="scirp.130907-ref2">2</xref>] and could rotate down to 30˚ during continuous activity [<xref ref-type="bibr" rid="scirp.130907-ref3">3</xref>] . In other words, the normal faults develop progressively, diminishing their dip as the extension proceeds. Because of fault rotation, the beds in faulted blocks also rotate. Drag develops where a layer is oriented at an angle to the slip vector of the fault. Two ways of bed rotation exist. One is the normal rotation, or normal drag, with beds on the hanging wall dipping in the same direction as that fault in the proximity of faults [<xref ref-type="bibr" rid="scirp.130907-ref4">4</xref>] (<xref ref-type="fig" rid="fig1">Figure 1</xref>(b)). Normal drag folds may form as a result of frictional drag [<xref ref-type="bibr" rid="scirp.130907-ref5">5</xref>] , differential compaction [<xref ref-type="bibr" rid="scirp.130907-ref6">6</xref>] , or due to fault-propagation [<xref ref-type="bibr" rid="scirp.130907-ref5">5</xref>] [<xref ref-type="bibr" rid="scirp.130907-ref7">7</xref>] [<xref ref-type="bibr" rid="scirp.130907-ref8">8</xref>] . Small normal drag folds may also form as a result of frictional drag along the fault surface [<xref ref-type="bibr" rid="scirp.130907-ref5">5</xref>] . For the normal drag, antiform is commonly found in the footwalls, whereas synform is observed in the hanging walls of normal faults. Another is the reverse rotation, or reverse drag, with beds on the hanging wall dipping in the opposite direction to the fault [<xref ref-type="bibr" rid="scirp.130907-ref9">9</xref>] .</p><p>Hamblin (1965) [<xref ref-type="bibr" rid="scirp.130907-ref4">4</xref>] listed several possible mechanisms of reverse drag including multiple stages of deformation, elastic rebound, sagging, inversion of slip directions and differential compaction. The convexity of fault drags may be used to ascertain the sense of slip along faults [<xref ref-type="bibr" rid="scirp.130907-ref10">10</xref>] . Reches and Eidelman (1995) [<xref ref-type="bibr" rid="scirp.130907-ref11">11</xref>] showed by means of numerical modeling that the drag varies from reverse drag at the fault center to normal drag at the termination of the fault. However, Graseman et al. (2005) [<xref ref-type="bibr" rid="scirp.130907-ref12">12</xref>] indicated that both normal and reverse drag can develop at the fault center depending on the angle between the markers and the fault; normal drag develops there for low angles (&lt;30˚ - 40˚) and reverse drag for higher angles (<xref ref-type="fig" rid="fig1">Figure 1</xref>). Drag folds have a smaller wavelength than reverse-drag or rollover folds and may be superimposed on these larger structures [<xref ref-type="bibr" rid="scirp.130907-ref5">5</xref>] .</p><p>Fault drag is common where there are ductile rocks in the faulted stratigraphy [<xref ref-type="bibr" rid="scirp.130907-ref13">13</xref>] [<xref ref-type="bibr" rid="scirp.130907-ref14">14</xref>] [<xref ref-type="bibr" rid="scirp.130907-ref15">15</xref>] . Drag folds are particularly important in hydrocarbon reservoirs where fault drag changes the communication path across a fault [<xref ref-type="bibr" rid="scirp.130907-ref16">16</xref>] [<xref ref-type="bibr" rid="scirp.130907-ref17">17</xref>] [<xref ref-type="bibr" rid="scirp.130907-ref18">18</xref>] [<xref ref-type="bibr" rid="scirp.130907-ref19">19</xref>] . Also, drag folds can provide useful information to assess earthquake hazards [<xref ref-type="bibr" rid="scirp.130907-ref20">20</xref>] . In this paper, we calculate fault displacement and related extension based on the reverse rotation of normal faults. Its purpose is to compare displacement and extension concerning the three mechanisms of reverse-drag flexures in an effort to determine the origin and significance of this type of structural deformation.</p></sec><sec id="s2"><title>2. Fault Total Displacement in the Case of Reverse Drag</title><sec id="s2_1"><title>2.1. Rigid Body Rotation</title><p>Reverse-drags are a result of the decrease in displacement with distance from the fault plane. There are three main explanations for bed tilting. One is based on the rigid body rotation model, which assumes that faults bound rigid blocks, the tilt angles of beds and faults are equal and there is no internal deformation within fault blocks (<xref ref-type="fig" rid="fig2">Figure 2</xref>(a) and <xref ref-type="fig" rid="fig2">Figure 2</xref>(b)). For this model, the angle of fault rotation (γ<sub>t1</sub>) is equal to bed tilt (θ),</p><p>γ t1 = δ 0 − δ 1 = θ (1)</p><p>where δ<sub>0</sub> is the original fault dip; δ<sub>1</sub> is the present dip of the fault.</p><p>For the rigid body rotation, any line between two faults is not altered, i.e. AB' = L<sub>0</sub>, where L<sub>0</sub> is the original horizontal distance between the two faults. The total displacement AD' can be calculated from triangle AB'D' by the law of sines.</p><p>D t1 = A D ′ = L 0 sin θ sin δ 1 = L 0 sin θ sin ( δ 0 − θ ) (2)</p><p>The above calculation is based on the fact that the basal detachment is horizontal. A more realistic case is that the basal detachment is inclined [<xref ref-type="bibr" rid="scirp.130907-ref21">21</xref>] . If the dip direction of the basal detachment is the same as that of faults, the detachment is synthetic (<xref ref-type="fig" rid="fig2">Figure 2</xref>(c) and <xref ref-type="fig" rid="fig2">Figure 2</xref>(d)). If the dip direction of the basal detachment is opposite to that of faults, the detachment is antithetical (<xref ref-type="fig" rid="fig2">Figure 2</xref>(e) and <xref ref-type="fig" rid="fig2">Figure 2</xref>(f)). Before rotation in the case of synthetic detachment, by using the law of sines for triangle ABC in <xref ref-type="fig" rid="fig2">Figure 2</xref>(c), we obtain</p><p>L t = AB = L 0 sin ( 180 − δ 0 ) sin ( δ 0 − α ) = L 0 sin δ 0 sin ( δ 0 − α ) (3)</p><p>Likewise, by applying the law of sines to triangle AA'B after rotation in <xref ref-type="fig" rid="fig2">Figure 2</xref>(d), the total displacement (AA') is determined by</p><p>D t 2 = A A ′ = L t sin θ sin ( δ 2 − α ) = L t sin θ sin ( δ 0 − θ − α ) = L 0 sin δ 0 sin θ sin ( δ 0 − α ) sin ( δ 0 − α − θ ) (4)</p><p>For this equation, the value of (α + θ) should be less than δ<sub>0</sub>, or the D<sub>t2</sub> is negative, which has no significance.</p><p>To compare D<sub>t1</sub> and D<sub>t2</sub>, let</p><p>λ 1 = D t2 / D t1 = sin δ 0 sin ( δ 0 − θ ) sin ( δ 0 − α ) sin ( δ 0 − θ − α ) (5)</p><p>Because the value of (α + θ) is less than the value of δ<sub>0</sub>, sin(δ<sub>0</sub>) is larger than sin(δ<sub>0</sub> − α) and sin(δ<sub>0</sub> − θ) is larger than sin(δ<sub>0</sub> − α − θ), which indicate that the value of λ<sub>1</sub> is larger than 1. That is to say, for the same values of δ<sub>0</sub> and θ, the synthetic basal detachment produce greater total displacement than the horizontal basal detachment (D<sub>t2</sub> &gt; D<sub>t1</sub>).</p><p>In the case of antithetic basal detachment, by using the law of sines for triangle ABC in <xref ref-type="fig" rid="fig2">Figure 2</xref>(e), we obtain</p><p>L t = AB = L 0 sin ( 180 − δ 0 ) sin ( 180 − δ 0 − α ) = L 0 sin δ 0 sin ( δ 0 + α ) (6)</p><p>After rotation, by applying the law of sines to triangle AA'B in <xref ref-type="fig" rid="fig2">Figure 2</xref>(f), the total displacement (BB') is determined by</p><p>D t 3 = B B ′ = L t sin θ sin ( δ 3 + α ) = L t sin θ sin ( δ 0 − θ + α ) = L 0 sin δ 0 sin θ sin ( δ 0 + α ) sin ( δ 0 + α − θ ) (7)</p><p>For this equation, the value of θ should be less than δ<sub>0</sub> + α, or the D<sub>t2</sub> is negative, which has no significance for normal faults.</p><p>To compare D<sub>t1</sub> and D<sub>t3</sub>, let</p><p>λ 2 = D t3 / D t1 = sin δ 0 sin ( δ 0 − θ ) sin ( δ 0 + α ) sin ( δ 0 − θ + α ) (8)</p><p>If the angle of the basal detachment α is less than 20˚ and δ<sub>0</sub> is equal to about 60˚, then (δ<sub>0</sub> + α) is less than 90˚. In this way sin (δ<sub>0</sub>) is less than sin(δ<sub>0</sub> + α) and sin(δ<sub>0</sub> − θ) is less than sin(δ<sub>0</sub> + α − θ). This indicates that the value of λ<sub>2</sub> is less than 1. In other words, for the same values of δ<sub>0</sub> and θ, the antithetic basal detachment produces smaller total displacement than the horizontal basal detachment (D<sub>t3</sub> &lt; D<sub>t1</sub>).</p></sec><sec id="s2_2"><title>2.2. Vertical Shear Model</title><p>The second model of rotation is the vertical shear mechanism proposed by Westaway and Kusznir (1993) [<xref ref-type="bibr" rid="scirp.130907-ref22">22</xref>] , who argued that the internal deformation within a block bounded by faults is due to vertical shear (<xref ref-type="fig" rid="fig3">Figure 3</xref>(a) and <xref ref-type="fig" rid="fig3">Figure 3</xref>(b)). According to the vertical shear model of Westaway and Kusznir (1993) [<xref ref-type="bibr" rid="scirp.130907-ref22">22</xref>] , if the bed tilt (θ), and present dip of the fault (δ<sub>4</sub>) are known, the original fault dip (δ<sub>0</sub>) are calculated by</p><p>tan δ 0 = tan θ + tan δ 4 (9)</p><p>Therefore, the fault rotation is</p><p>γ t2 = δ 0 − δ 4 = arctan ( tan θ + tan δ 4 ) − δ 4 (10)</p><p>The vertical simple shear causes the initially horizontal surface between faults to be progressively tilted by an angle θ. Thus, the final length (L<sub>b</sub>) of a bed is AB'. Using trigonometry,</p><p>L b = A B ′ = L 0 / cos θ (11)</p><p>Then, according to the law of sines the total displacement (D<sub>t4</sub>) is</p><p>D t 4 = A D ′ = L 0 sin θ sin δ 4 cos θ (12)</p><p>Combining Equations (2) and (12), we define</p><p>λ 3 = D t4 / D t1 = sin δ 1 sin δ 4 cos θ = sin ( δ 0 − θ ) sin [ arctan ( tan δ 0 − tan θ ) ] cos θ (13)</p><p>This Equation (13) indicates that the value of D<sub>t4</sub>/D<sub>t1</sub> is dependent on the original fault dip δ<sub>0</sub> and bedding angle θ. For given value of δ<sub>0</sub>, the value of λ<sub>3</sub> is negatively related to the value of θ (<xref ref-type="fig" rid="fig3">Figure 3</xref>(c)). For a given value of θ, the curve of λ<sub>3</sub> shows an up convex (<xref ref-type="fig" rid="fig3">Figure 3</xref>(d)). In all cases, the value of λ<sub>3</sub> is less than 1, which indicates the value of D<sub>t4</sub> is smaller than D<sub>t1</sub> (D<sub>t4</sub> &lt; D<sub>t1</sub>).</p></sec><sec id="s2_3"><title>2.3. Inclined Shear Model</title><p>The third model of rotation is the inclined shear model [<xref ref-type="bibr" rid="scirp.130907-ref23">23</xref>] [<xref ref-type="bibr" rid="scirp.130907-ref24">24</xref>] , which proposes that the internal deformation within blocks is due to arbitrary oblique shear. To compare this model with the other two models, we first determine the relationship between the final dip and original dip of the fault. The geometrical configuration is shown in <xref ref-type="fig" rid="fig4">Figure 4</xref>. The original normal fault is expressed by the equation:</p><p>z = a x + b (14)</p><p>For this equation tan(δ<sub>0</sub>) = −a.</p><p>The relationship between fault and bed tilting by ideal inclined shear in the limit of infinitesimal distance from the fault is shown in <xref ref-type="fig" rid="fig4">Figure 4</xref>(c). In this condition, the fault is assumed to remain planar, with local dip δ<sub>5</sub>. After extension (<xref ref-type="fig" rid="fig4">Figure 4</xref>(b)) the initially horizontal marker has equation:</p><p>y = c x + d 2 (15)</p><p>where c = tanθ. An adjacent point D on the fault is now located in point A. Now the vertical coordinate u along DB is</p><p>u = z + DB (16)</p><p>For right triangle ACD, AC = y, and &#208;CAD = β, which is the angle of inclined shear direction, then AD = y/cosβ. By applying the law of sines to triangle ADB, we obtain that</p><p>DB = AD sin ( 90 − θ − β ) sin ( 90 + θ ) = y cos ( θ + β ) cos θ cos β = y ( 1 − tan θ tan β ) (17)</p><p>Substituting DB into Equation (17) gives</p><p>u = z + DB = a x + b + ( c x + d 2 ) ( 1 − tan θ tan β ) = ( a + c ( 1 − tan θ tan β ) ) x + b + d 2 ( 1 − tan θ tan β ) (18)</p><p>After the inclined shear, the fault dip δ<sub>5</sub> will satisfy that tan(δ<sub>5</sub>) = −(a + c(1 - tanθtanβ)). Substituting for a and c gives:</p><p>tan δ 5 = tan δ 0 − tan θ ( 1 − tan θ tan β ) , (19)</p><p>Thus,</p><p>tan δ 0 = tan δ 5 + tan θ ( 1 − tan θ tan β ) (20)</p><p>For β = 0, this equation is the same as Equation (9), which indicates that the vertical shear is the case of end member of the inclined shear. Compare this Equation (20) with Equation (9) that assumes vertical shear, we can see that for the same present fault dip (δ<sub>5</sub> = δ<sub>4</sub>) and bedding tilt θ, the calculated original fault dip for inclined shear is smaller than that for the vertical shear. The difference of initial fault dips between the two shear models (Δδ) is</p><p>Δ δ = arctan ( tan 2 θ tan β ) (21)</p><p>This equation indicates that the value of Δδ is positively related to the bedding tilt θ and the angle of inclined shear (β). The effects of the bedding tilt θ and the angle of inclined shear β are shown in <xref ref-type="fig" rid="fig5">Figure 5</xref>.</p><p>The value of β is negative if measured from vertical line to shear direction (<xref ref-type="fig" rid="fig4">Figure 4</xref>(c)). As shown in <xref ref-type="fig" rid="fig4">Figure 4</xref>(d), DB' is the initial length of bedding (L<sub>0</sub>). Based on the law of sinces for triangle ADB', the present length (L<sub>b</sub> = AB') of the bedding is:</p><p>L b = A B ′ = L 0 sin ( 90 + β ) sin ( 90 − β − θ ) = L 0 cos β cos ( β + θ ) (22)</p><p>By applying the law of sines to triangle AD'B', the total displacement AD' can be deduced as</p><p>D t5 = A D ′ = L 0 sin θ cos β sin δ 5 cos ( β + θ ) = L 0 sin θ cos β sin { arctan [ tan δ 0 − tan θ ( 1 − tan θ tan β ) ] } cos ( β + θ ) (23)</p><p>To compare D<sub>t5</sub> with D<sub>t1</sub>, let λ<sub>4</sub> = D<sub>t5</sub>/D<sub>t1</sub>. Combining Equations (2) and (23), we obtain</p><p>λ 4 = D t5 / D t1 = cos β sin ( δ 0 − θ ) sin [ arctan ( tan δ 0 − tan θ ( 1 − tan θ tan β ) ) ] cos ( β + θ ) (24)</p><p>Equation (24) shows that the value of D<sub>t5</sub>/D<sub>t1</sub> is dependent on the original fault dip δ<sub>0</sub> and bedding angle θ, and the angle of shear direction β. Generally, the value of λ<sub>4</sub> can be either less than or larger than 1 (<xref ref-type="fig" rid="fig6">Figure 6</xref>), which indicates D<sub>t5</sub> &lt; D<sub>t1</sub> or D<sub>t5</sub> &gt; D<sub>t1</sub>. For given values of δ<sub>0</sub> and θ, the value of λ<sub>4</sub> is positively related to the value of β (<xref ref-type="fig" rid="fig6">Figure 6</xref>(a)). For given values of δ<sub>0</sub> and β, there are negative relationship between the value of λ<sub>4</sub> and the value of θ (<xref ref-type="fig" rid="fig6">Figure 6</xref>(b)). Finally, for given values of θ and β, the value of λ<sub>4</sub> is also negatively related to the value of δ<sub>0</sub> (<xref ref-type="fig" rid="fig6">Figure 6</xref>(c)).</p><p>Let λ<sub>5</sub> = D<sub>t5</sub>/D<sub>t4</sub> to compare the value of D<sub>t5</sub> with D<sub>t4</sub>. Based on Equations (12) and (23), we have</p><p>λ 5 = D t5 / D t4 = cos β cos θ sin [ arctan ( tan δ 0 − tan θ ) ] sin [ arctan ( tan δ 0 − tan θ ( 1 − tan θ tan β ) ) ] cos ( β + θ ) (25)</p><p>There are three variables in this equation: the original fault dip δ<sub>0</sub>, the bedding angle θ, and the angle of shear direction β. For any combination of these three variables, the value of λ<sub>5</sub> is always smaller than 1 (<xref ref-type="fig" rid="fig7">Figure 7</xref>). This indicates that for same condition, the inclined shear produces less displacement than the vertical shear. The original fault dip δ<sub>0</sub> does not strongly affect the value of λ<sub>5</sub> (<xref ref-type="fig" rid="fig7">Figure 7</xref>(a)). When the value of δ<sub>0</sub> varies from 70˚ to 30˚, the difference of λ<sub>5</sub> is only equal to 0.04 changing from 0.72 to 0.78. On the other hand, the value of λ<sub>5</sub> is negatively related to the value of θ, and is positively related to the value of β (<xref ref-type="fig" rid="fig7">Figure 7</xref>(b) and <xref ref-type="fig" rid="fig7">Figure 7</xref>(c)).</p></sec></sec><sec id="s3"><title>3. Horizontal Extension in the Case of Reverse Drag</title><p>For any model of block rotation, the horizontal extension is defined as</p><p>ε = ( L f − L 0 ) / L 0 (26)</p><p>where L<sub>f</sub> is the final horizontal length of the fault block after faulting, and L<sub>0</sub> is the initial horizontal length. For the rigid body mechanism on the horizontal base (<xref ref-type="fig" rid="fig2">Figure 2</xref>(a) and <xref ref-type="fig" rid="fig2">Figure 2</xref>(b)), the fault heave is h<sub>1</sub> = D<sub>t1</sub>cosδ<sub>1</sub> = D<sub>t1</sub>cos(δ<sub>0</sub> − θ). Substituting for D<sub>t1</sub> from Equation (2), we have h<sub>1</sub> = L<sub>0</sub>ctan(δ<sub>0</sub> − θ)sinθ. Then, the final length is L<sub>f1</sub> = h<sub>1</sub> + DB' = L<sub>0</sub>[cot(δ<sub>1</sub>)sin(θ) + cos(θ)]. In this way, the extension is expressed as</p><p>ε 1 = ( L f1 − L 0 ) / L 0 = ctan ( δ 0 − θ ) sin θ + cos θ − 1 (27)</p><p>For the vertical shear, the horizontal distance between the footwall cutoff of one fault and the hanging-wall cutoff in the next fault will remain constant over time, having its initial value (L<sub>0</sub> in <xref ref-type="fig" rid="fig3">Figure 3</xref>(b)). In this way, we have DB' = L<sub>0</sub> and the heave is h<sub>2</sub> = D'D = D<sub>t4</sub>cosδ<sub>4</sub>. The value of D<sub>t4</sub> is substituted into this equation, we have h<sub>2</sub> = L<sub>0</sub>ctanδ<sub>4</sub>tanθ, then L<sub>f4</sub> = DB' + D'D = L<sub>0</sub>(ctanδ<sub>4</sub>tanθ + 1) = L<sub>0</sub>(ctan(arctan(tanδ<sub>0</sub> − tanθ))tanθ + 1) = L<sub>0</sub>(tanθ/(tanδ<sub>0</sub> − tanθ) + 1). Therefore, the horizontal extension is</p><p>ε 4 = ( L f4 − L 0 ) / L 0 = tan θ / ( tan δ 0 − tan θ ) (28)</p><p>To compare the value of ε<sub>4</sub> with ε<sub>1</sub>, let ρ<sub>1</sub> = ε<sub>4</sub>/ε<sub>1</sub>. From Equations (27) and (28), we deduce</p><p>ρ 1 = ε 4 / ε 1 = tan θ ( tan δ 0 − tan θ ) ( ctan ( δ 0 − θ ) sin θ + cos θ − 1 ) (29)</p><p>This equation indicates that the value of ε<sub>2</sub>/ε<sub>1</sub> is dependent on the original fault dip δ<sub>0</sub> and bedding angle θ. For given values of δ<sub>0</sub>, the value of ρ<sub>1</sub> is negatively related to the value of θ (<xref ref-type="fig" rid="fig8">Figure 8</xref>(a)). For given value of θ, the value of ρ<sub>1</sub> is also negatively related to the value of δ<sub>0</sub> (<xref ref-type="fig" rid="fig8">Figure 8</xref>(b)). For all combinations of fault dip δ<sub>0</sub> and bedding angle θ, the value of ρ<sub>1</sub> is less than 1, which indicates the value of ε<sub>4</sub> is smaller than ε<sub>1</sub> (ε<sub>4</sub> &lt; ε<sub>1</sub>). Note that for the same values of fault dip δ<sub>0</sub> and bedding angle θ, the value of ρ<sub>1</sub> is not equal to λ<sub>3</sub>, that is, ε<sub>4</sub>/ε<sub>1</sub> &#185; D<sub>t4</sub>/D<sub>t1</sub>. For example, given δ<sub>0</sub> = 60˚ and θ = 20˚, ρ<sub>3</sub> = 0.766, whereas λ<sub>3</sub> = 0.847.</p><p>Now we consider the inclined shear in <xref ref-type="fig" rid="fig4">Figure 4</xref>(d), the value of final length L<sub>f5</sub> = D'D + DB' = D'C + CD + L<sub>0</sub>. Because D'C = D<sub>t5</sub>cosδ<sub>5</sub> and CD = ACtanβ = D'Ctanδ<sub>5</sub>tanβ, L<sub>f5</sub> = D<sub>t5</sub>cosδ<sub>5</sub>(1 + tanδ<sub>5</sub>tanβ) + L<sub>0</sub>. Substituting for D<sub>t5</sub> gives</p><p>L f5 = L 0 ( sin θ cos β ( 1 + tan δ 5 tan β ) tan δ 5 cos ( β + θ ) + 1 ) (30)</p><p>Thus, the extension is</p><p>ε 5 = L f 5 − L 0 L 0 = sin θ cos β ( 1 + tan δ 5 tan β ) tan δ 5 cos ( β + θ ) (31)</p><p>Apply the equation cos(β + θ) = cosβcosθ − sinβsinθ to Equation (31) and rearrange it, we obtain</p><p>ε 5 = tan θ ( 1 + tan δ 5 tan β ) tan δ 5 ( 1 − tan θ tan β ) (32)</p><p>where tanδ<sub>5</sub> is calculated by Equation (19), which depends on the values of δ<sub>0</sub> and β. We define ρ<sub>2</sub> = ε<sub>5</sub>/ε<sub>1</sub>, the expression of ρ<sub>2</sub> is</p><p>ρ 2 = tan θ ( 1 + tan δ 5 tan β ) tan δ 5 ( 1 − tan θ tan β ) ( c tan ( δ 0 − θ ) sin θ + cos θ + 1 ) (33)</p><p>For all combinations of fault dip δ<sub>0</sub>, the angle of inclined shear β, and bedding angle θ, the value of ρ<sub>2</sub> can be less than or greater than 1 (<xref ref-type="fig" rid="fig9">Figure 9</xref>). For given value of θ, the value of ρ<sub>2</sub> is negatively related to the value of δ<sub>0</sub> (<xref ref-type="fig" rid="fig9">Figure 9</xref>(a)). On the other hand, the value of ρ<sub>2</sub> is positively related to the value of θ and β (<xref ref-type="fig" rid="fig9">Figure 9</xref>(b) and <xref ref-type="fig" rid="fig9">Figure 9</xref>(c)). Compare <xref ref-type="fig" rid="fig8">Figure 8</xref> with Figures 10(a)-(c), there are two differences between ρ<sub>2</sub> and λ<sub>4</sub>. First, the range of ρ<sub>2</sub> is larger than that of λ<sub>4</sub>. The value of λ<sub>4</sub> is less than 2, whereas the value of ρ<sub>2</sub> can be greater than 10. Second, the curves of ρ<sub>2</sub> and λ<sub>4</sub> have opposite tendency related to δ<sub>0</sub>, θ and β.</p><p>On the other hand, we define ρ<sub>3</sub> = ε<sub>5</sub>/ε<sub>4</sub>, the expression of ρ<sub>3</sub> is</p><p>ρ 3 = ( tan δ 0 − tan θ ) ( 1 + tan δ 5 tan β ) tan δ 5 ( 1 − tan θ tan β ) (34)</p><p>There are three variables in this equation: the original fault dip δ<sub>0</sub>, the bedding angle θ, and the angle of shear direction β. For any combination of these three variables, the value of ρ<sub>3</sub> is always larger than 1 (Figures 9(d)-(f)). This indicates that for same condition, the inclined shear produces larger extension than the vertical shear. On the other hand, the value of ρ<sub>3</sub> is positively related to the value of β and δ<sub>0</sub> (<xref ref-type="fig" rid="fig9">Figure 9</xref>(d) and <xref ref-type="fig" rid="fig9">Figure 9</xref>(f)). For given value of β and δ<sub>0</sub>, the curve of ρ<sub>3</sub> shows a down convex curve (<xref ref-type="fig" rid="fig9">Figure 9</xref>(e)). Compare <xref ref-type="fig" rid="fig7">Figure 7</xref> and Figures 9(d)-(f), one can see the curve of λ<sub>5</sub> and ρ<sub>3</sub> show different tendency. The values of λ<sub>5</sub> and ρ<sub>3</sub> are quite different: λ<sub>5</sub> &lt; 1, whereas ρ<sub>3</sub> &gt; 1. This indicates although the displacement caused by the inclined shear is smaller than that by the vertical shear, the extension from the inclined shear is larger than from the vertical shear.</p></sec><sec id="s4"><title>4. Discussion</title><p>The rigid body model indicates that there is no internal deformation within the blocks. The simple shear models represent internal deformation. For listric normal faults, there are some other techniques to estimate horizontal extension. Exception for the simple shear method, Poblet and Bulnes (2005) [<xref ref-type="bibr" rid="scirp.130907-ref25">25</xref>] discussed six geometric mechanisms to estimate horizontal extension in simple listric normal faults. They are: 1) unfolding sinuous bed lengths, e = structural width-folded bed length; 2) equal-area calculation, e = dropped area/detachment depth; 3) heave, e = heave; 4) maximum displacement on the fault, e = displacement on the fault; 5) extensional fault-bend folding, e = distance between hanging-wall active and inactive axial surfaces along the lower flat; and 6) lost-area diagram, e = slop of best-fit line across area versus depth of several horizons to a reference level. By physical experiments, they showed that the estimated extension from different methods yield different magnitudes. Although the maximum displacement method and the extensional fault-bend folding method yield the best estimates of the amount of horizontal extension, all estimated extensions are less than the true values. These results indicate that hidden extensions may occur on faults below the smaller scale during progressive deformation [<xref ref-type="bibr" rid="scirp.130907-ref26">26</xref>] .</p><p>Estimation of extension of field examples from the Norwegian margin fault and the Gulf Coast fault are shown in <xref ref-type="fig" rid="fig1">Figure 1</xref>0. Different methods yield evident different results. In both examples, the maximum displacement and inclined shear methods have the largest amounts of extension. Here, the values of fault shear used to apply the inclined shear are 40˚ for the Norwegian margin fault, and 20˚ for Gulf Coast fault. These results are different from the results of physical experiments [<xref ref-type="bibr" rid="scirp.130907-ref25">25</xref>] .</p></sec><sec id="s5"><title>5. Conclusions</title><p>For reverse drags, three mechanisms were considered in our analysis: rigid body rotation, vertical shear, and inclined shear. The results allow us to obtain the following conclusions.</p><p>The established equations show that among the three subcases of rigid body model, the total displacement in the case of the synthetic base (D<sub>t2</sub>) is the largest, that in the case of the horizontal base (D<sub>t1</sub>) is moderate, and that in the case of the antithetic base (D<sub>t3</sub>) is the smallest based on the same original parameters. Also, it is shown that the value of (D<sub>t1</sub>) is larger than the displacement for the vertical shear (D<sub>t4</sub>). the displacement for the inclined shear (D<sub>t5</sub>) can be either larger than or less than the value of (D<sub>t1</sub>) depending on combination among the original fault dip δ<sub>0</sub>, bedding angle θ, and the angle of shear direction β. For all original parameters, the value of D<sub>t5</sub> is less than the value of D<sub>t4</sub>. As for horizontal extension due to normal faulting, we find that the inclined shear produces largest value, the rigid body model with horizontal detachment produces the smallest value, and the vertical shear model produces moderate value.</p></sec><sec id="s6"><title>Acknowledgements</title><p>This work was supported by the PAPIIT project IN102020 of National University of Mexico, the project 2020B1515020016 (Natural Science Foundation for Youths of Guangdong), and the project 2022YFC3102200 (National Key Research and Development Program of China).</p></sec><sec id="s7"><title>Conflicts of Interest</title><p>The authors declare no conflicts of interest regarding the publication of this paper.</p></sec><sec id="s8"><title>Cite this paper</title><p>Xu, S.S., Nieto-Samaniego, A.F., Xu, H.L. and Alaniz-&#193;lvarez, S.A. (2024) Estimation of Displacement and Extension due to Reverse Drag of Normal Faults: Forward Method. International Journal of Geosciences, 15, 25-39. https://doi.org/10.4236/ijg.2024.151003</p></sec></body><back><ref-list><title>References</title><ref id="scirp.130907-ref1"><label>1</label><mixed-citation publication-type="other" xlink:type="simple">Anderson, E.M. (1951) The Dynamic of Faulting and Dyke Formation. Oliver and Boyd, Edinburgh, 206 p.</mixed-citation></ref><ref id="scirp.130907-ref2"><label>2</label><mixed-citation publication-type="other" xlink:type="simple">Jaeger, J.C., Cook, N.G.W. and Zimmerman, R.W. (2007) Fundamentals of Rock Mechanics. Blackwell Publishing, Hoboken.</mixed-citation></ref><ref id="scirp.130907-ref3"><label>3</label><mixed-citation publication-type="other" xlink:type="simple">Sibson, R.H. (1985) A Note on Fault Reactivation. 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